1. Field of the Invention
The present invention relates to wireless communications and, more specifically but not exclusively, to cellular communications systems that can operate in a distributed diversity mode in which multiple base stations transmit the same data to a single user.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
In order to realize the enormous data capacity demand of smart mobile devices and meet user quality-of-service requirements, cellular communications systems having low-power, low-cost, small cells have been widely considered as one of the most-promising solutions. However, there are three major limitations: 1) when additional small cells are added to the system to improve capacity, the cell-edge users (i.e., users located at or near the edge of one cell that is adjacent to another cell) can experience a lot of strong interference levels degrading the throughput performance and actual user experience; 2) each cell is costly to build and operate; and 3) the traffic at each cell fluctuates over time such that the cell may have to be designed to handle the maximum traffic expected, causing a waste of processing resources at off-peak times.
Traditionally, small-cell architectures are proposed to solve the capacity problem. As the density of a small-cell network increases, the interference increases. Traditionally, interference mitigation is achieved by the following techniques:
a) Power control: reduce power to meet desirable SINR (signal-to-interference-noise ratio) target;
b) Marco diversity: cell-edge user communicates with two or more base stations (BSs) simultaneously using a distributed diversity mode in which multiple base stations transmit the same data to a single user; and                c) Fractional frequency reuse: segmenting the spectrum into several frequency bands and enabling allocation of different frequency bands to different users.        
Without losing any generality, the Long Term Evolution (LTE) communications standard is taken as a reference technology for this disclosure. Those skilled in the art will understand that the disclosure can also be applied in the context of other suitable communications standards.
In LTE-A, coordinated multipoint (CoMP) transmission and reception techniques are considered as promising candidates for efficient interference management to improve cell-edge users' performance. There are different CoMP schemes with different levels of Channel State Information (CSI) sharing and estimation requirements.
In coherent joint transmission, at least two BSs perform joint Multiple-Input Multiple-Output (MIMO) transmission to multiple user equipments (UEs, also referred to herein simply as users) located in different cells. (The terms “cell” and “base station” or BS are used interchangeably in this disclosure.) Although this type of transmission scheme achieves good performance, it has a high backhaul requirement because CSI exchange is required between BSs. This is especially true for frequency-selective channels, where the channel has to be estimated and exchanged for each orthogonal frequency-division multiplexing (OFDM) sub-carrier.
In a non-coherent joint transmission (JT) scheme, a precoder is calculated from each base station independently, based on the local CSI information between the BS and the UE. CSI exchanges among base stations is not required for this type of transmission scheme; however, the performance is typically not as good as the one offered by the coherent joint transmission scheme. It should be noted that, even though CSI exchange is not required between BSs, each BS still has to estimate the channel to the cell-edge user independently.
The aforementioned CoMP schemes use multiple BSs to either beamform the source signal or transmit the source signal using spatial multiplexing. In Marco Diversity (also referred to herein as distributed diversity), multiple BSs transmit the same data to the cell-edge user. No CSI exchange between BSs is required. Due to the different geographical locations of the BSs, received signals at any UE location are most of the time not synchronized. However, as long as the cyclic prefix (CP) is longer than the maximum delay spread from the BSs, the channel at each frequency tone/sub-carrier can still be considered as flat. Essentially, by transmitting the same data from multiple BSs and the fact that the data arrives at different time instances, the resulting channel is effectively turned into a more-frequency-selective channel with a longer delay spread. The increased frequency-selectivity of the channel can be exploited with a forward error-correcting channel code that encodes across several frequency blocks, e.g., LTE Physical Resource Blocks (PRBs).
A key feature of LTE is the possibility to exploit the OFDM radio interface to transmit multicast or broadcast data as multi-cell transmission over a synchronized single-frequency network known as a multicast-broadcast, single-frequency network (MBSFN). In MBSFN, multimedia broadcast/multicast service (MBMS) data is transmitted from multiple time-synchronized cells simultaneously. A UE will receive multiple versions of the signal with different delays. There will be no Inter-Symbol Interference (ISI) if the cyclic prefix is longer than the maximum delay spread of the equivalent channel. It should be noted that MBSFN is a broadcast/multicast feature, where multiple BSs transmit the same information to multiple UEs. Therefore, it is not suitable for unicast transmission, where BSs need to support independent communications with multiple UEs simultaneously.
The above solutions are not sufficient because the cost of increasing the density of small-cell architectures is high. It is also inefficient because traffic is likely bursty in time, resulting in processing resources being wasted when traffic is light.